Whether large ferroelectric polarization P exists in the orthorhombic HoMnO 3 with the E-type antiferromagnetic spin ordering or not remains as one of unresolved, challenging issues in the physics of multiferroics. The issue is closely linked to an intriguing experimental difficulty for determining P of polycrystalline specimens that conventional pyroelectric current measurements performed after a poling procedure under high dc electric fields are subject to large errors due to the problems caused by leakage currents or space charges. To overcome the difficulty, we employed the PUND method, which uses successively the two positive and two negative electrical pulses, to directly measure electrical hysteresis loops in several polycrystalline HoMnO 3 specimens below their Néel temperatures. We found that all the * These authors equally contributed to this work. HoMnO 3 samples had similar remnant polarization P r values at each temperature, regardless of their variations in resistivity, dielectric constant, and pyroelectric current levels. Moreover, P r of~0.07 µC/cm 2 at 6 K is consistent with the P value obtained from the pyroelectric current measurement performed after a short pulse poling. Our findings suggest that intrinsic P of polycrystalline HoMnO 3 can be determined through the PUND method and P at 0 K may reach~0.24 µC/cm 2 in a single crystalline specimen.

In order to investigate whether magnetism and superconductivity coexist in Co-doped SrFe(2)As(2), we have prepared single crystals of SrFe(2-x)Co(x)As(2), x = 0 and 0.4, and characterized them via x-ray diffraction, electrical resistivity in zero and applied field up to 9 T as well as at ambient and applied pressure up to 1.6 GPa, and magnetic susceptibility. At x = 0.4, there is both magnetic and resistive evidence for a spin density wave transition at 120 K, while T(c) = 19.5 K-indicating coexistent magnetism and superconductivity. A discussion of how these results compare with reported results, both in SrFe(2-x)Co(x)As(2) and in other doped 122 compounds, is given.

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By directly measuring electrical hysteresis loops using the Positive-Up Negative-Down (PUND) method, we determined accurately the remanent ferroelectric polarization P r of orthorhombic RMnO 3 (R = Ho, Tm, Yb, and Lu) compounds below their E-type spin ordering temperatures. We found that LuMnO 3 has the largest P r of 0.17 μC/cm 2 at 6 K in the series, the value of which allows us to predict that its single-crystal form can produce a P r of at least 0.6 μC/cm 2 at 0 K. Furthermore, at a fixed temperature, P r decreases systematically with increasing rare earth ion radius from R = Lu to Ho, exhibiting a strong correlation with the variation of the in-plane Mn-O-Mn bond angle and Mn-O distances. Our experimental results suggest that the contribution of the Mn t 2g orbitals may dominate the ferroelectric polarization.

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